mdcache: fix comment typo
mdcache: fix comment typo

Suggested by songgao.

Issue: #1075

keybase/kbfs,keybase/client,keybase/client,keybase/kbfs,keybase/kbfs,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client